U.S. patent application number 16/695844 was filed with the patent office on 2020-05-28 for process for producing hydrogen by steam reforming and conversion of co.
This patent application is currently assigned to L'Air Liquide, Societe Anonyme pour l'Etude et l?Exploitation des Procedes Georges Claude. The applicant listed for this patent is L'Air Liquide, Societe Anonyme pour I'Etude et I'Exploitation des Procedes Georges Claude. Invention is credited to Marie BASIN, Daniel GARY, Jean-Philippe TADIELLO.
Application Number | 20200165128 16/695844 |
Document ID | / |
Family ID | 66166165 |
Filed Date | 2020-05-28 |
United States Patent
Application |
20200165128 |
Kind Code |
A1 |
BASIN; Marie ; et
al. |
May 28, 2020 |
PROCESS FOR PRODUCING HYDROGEN BY STEAM REFORMING AND CONVERSION OF
CO
Abstract
The invention relates to a process for producing hydrogen from a
light hydrocarbon source, in which a synthesis gas is generated by
steam methane reforming after desulfurization and optionally
pre-reforming of the feedstock. The synthesis gas is enriched with
hydrogen by steam conversion of carbon monoxide, and is
subsequently purified in a pressure swing adsorption unit to give a
pure H.sub.2 product and a residual gas mixture containing
CH.sub.4, CO, H.sub.2 and CO.sub.2; in accordance with the
invention, the conversion step is performed in a cooled reactor in
which the heat of the conversion reaction is transferred to a fluid
which feeds the burners of the reformer, or to the gas for
reforming.
Inventors: |
BASIN; Marie; (Paris,
FR) ; GARY; Daniel; (Paris, FR) ; TADIELLO;
Jean-Philippe; (Frankfurt am Main, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
L'Air Liquide, Societe Anonyme pour I'Etude et I'Exploitation des
Procedes Georges Claude |
Paris |
|
FR |
|
|
Assignee: |
L'Air Liquide, Societe Anonyme pour
l'Etude et l?Exploitation des Procedes Georges Claude
Paris
FR
|
Family ID: |
66166165 |
Appl. No.: |
16/695844 |
Filed: |
November 26, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01B 2203/0233 20130101;
B01J 8/067 20130101; C01B 2203/1258 20130101; C01B 2203/1241
20130101; C01B 3/38 20130101; C01B 3/48 20130101; C01B 2203/82
20130101; B01J 8/0496 20130101; C01B 2203/1247 20130101; B01J 8/062
20130101; C01B 2203/043 20130101; C01B 2203/0283 20130101; C01B
2203/0811 20130101; C01B 2203/0883 20130101; C01B 3/56 20130101;
C01B 2203/1614 20130101 |
International
Class: |
C01B 3/48 20060101
C01B003/48; B01J 8/04 20060101 B01J008/04 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 27, 2018 |
FR |
FR 1871927 |
Claims
1. A process for producing hydrogen from a light hydrocarbon
source, comprising at least: (a) generating a synthesis gas by
steam reforming of said light gaseous hydrocarbons, comprising at
least a step (a1) of desulfurizing said hydrocarbons for reforming,
a step of steam-reforming the desulfurized hydrocarbons in tubular
reactors installed in a furnace of a reformer heated by burners
which are fed with fuel by at least a secondary fuel gas, (b)
producing hydrogen from the synthesis gas generated in step (a),
comprising at least a step (b1) of hydrogen-enriching the synthesis
gas by conversion of carbon monoxide according to the exothermic
conversion reaction CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2 and a step
(b2) of purifying the synthesis gas in a pressure swing adsorption
unit to give a pure H.sub.2 product and a residual gas mixture
containing CH.sub.4, CO, H.sub.2 and CO.sub.2, wherein the
secondary fuel gas is comprised of at least some of the residual
gas mixture produced in the hydrogen production step (b) and a
primary fuel gas, wherein the conversion reaction of step (b1) is
carried out in a cooled conversion reactor in which some of the
heat produced in step (b1) is transferred by indirect heat exchange
in the reactor with a first gaseous fluid so as to preheat the
first gaseous fluid before the first gas fluid is used in the
synthesis gas generation step (a), and in that the
hydrogen-enriched synthesis gas leaving the conversion reactor is
cooled by indirect heat exchange with a second gaseous fluid before
the second gaseous fluid is used in step (a) as the secondary fuel
gas.
2. The process according to claim 1, wherein the synthesis gas
enters the conversion reactor at a temperature T.sub.E, the
hydrogen-enriched synthesis gas leaves the reactor cooled to a
temperature T.sub.S of less than T.sub.E+40.degree. C., preferably
less than T.sub.E+30.degree. C., more preferably less than
T.sub.E+10.degree. C.
3. The process according to claim 1, wherein the first gaseous
fluid for preheating is the residual gas mixture feeding the
burners of the reformer, and the second gaseous fluid for
preheating is composed of the light hydrocarbons for reforming.
4. The process according to claim 1, wherein the first gaseous
fluid for preheating is composed of the light hydrocarbons for
reforming and the second gaseous fluid for preheating is the
residual gas mixture feeding the burners of the reformer.
5. The process according to claim 1, wherein for a hydrogen
production level and operating conditions which are otherwise
similar, the total consumption of light hydrocarbons is decreased
by at least 1%, preferably at least 2%, more preferably still at
least 3% relative to a process employing an adiabatic conversion
reactor.
6. A cooled conversion reactor suitable for implementation of the
process as defined in claim 1, wherein the cooled conversion
reactor is configured to admit the first fluid for preheating
before the first fluid is used in step (a), wherein the cooled
conversion reactor comprises reactor-internal means for circulating
said first fluid for preheating and for heat exchange with the
synthesis gas for cooling, and means for exit of said preheated
first fluid.
7. The cooled conversion reactor according to claim 6, wherein the
cooled conversion reactor is a plate-corrugated sheet reactor.
8. The cooled conversion reactor according to claim 6, wherein the
cooled conversion reactor is a shell-and-tube reactor and is
adapted for circulating the synthesis gas in catalyst-filled tubes
and for circulating said first fluid for preheating in the
shell.
9. The cooled conversion reactor according to claim 6, wherein the
cooled conversion reactor is a shell-and-tube reactor and is
adapted for circulating the synthesis gas in the catalyst-filled
shell and for circulating said first fluid for preheating in the
tubes.
10. A plant suitable for implementing the process according to
claim 1 wherein the plant comprises: a cooled conversion reactor,
wherein the cooled conversion reactor is configured to admit the
first fluid for preheating before the first fluid is used in step
(a), wherein the cooled conversion reactor comprises
reactor-internal means for circulating said first fluid for
preheating and for heat exchange with the synthesis gas for
cooling, and means for exiting of said preheated first fluid; and
means for conveying said first fluid for preheating before said
first fluid is used in step (a) and for bringing the first fluid to
the said reactor; and means for conveying said first preheated
fluid from its exit from said reactor to the location of its use at
the reforming stage.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn. 119 (a) and (b) to French patent application No.
FR1871927, filed Nov. 27, 2018, the entire contents of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a process for producing
hydrogen from a light hydrocarbon source, in which a synthesis gas
is generated by steam methane reforming, comprising a step of
desulfurizing the hydrocarbons for reforming, followed by an
optional pre-reforming step, a step of steam-reforming the
desulfurized and pre-reformed hydrocarbons, the hydrogen being
produced from the synthesis gas by hydrogen enrichment of the
synthesis gas by conversion of carbon monoxide and purification in
a pressure swing adsorption unit to give a pure H.sub.2 product and
a residual gas mixture containing CH.sub.4, CO, H.sub.2 and
CO.sub.2.
BACKGROUND OF THE INVENTION
[0003] Synthesis gas (also called syngas) is currently still
produced predominantly by steam reforming of methane (steam methane
reforming). The feedstock for the reformer is composed of light
gaseous hydrocarbons (with gaseous referring to gaseous or
vaporized liquid hydrocarbons); it usually comprises natural gas,
methane, propane, butane, naphtha, and also light hydrocarbons
obtained from certain refinery residuals, which are used alone or
in combination; in the context of the invention, the expression
"light hydrocarbons" will therefore include the refinery residuals
employed as a source for the reforming. The feedstock will
generally be pre-treated, i.e. desulfurized and if necessary
pre-reformed, in order to convert the heavier hydrocarbons into
methane before the steam reforming step itself. At the end of the
reforming, a synthesis gas is obtained in which the two principal
constituents are hydrogen and carbon monoxide but which also
includes carbon dioxide, excess steam, residual methane and
impurities. A small part of the hydrocarbons making up the
feedstock is often used as a primary fuel for the reforming;
accordingly, the process feed gas is differentiated from the fuel
gas.
[0004] For simplification, and bearing in mind that the most
commonly used source of light gaseous hydrocarbons is natural gas
(or NG), reference will be made to natural gas rather than to
hydrocarbons; in the remainder of the text, therefore, the process
natural gas (also called process NG) is differentiated from the
fuel natural gas (also called fuel NG), on the understanding that
this description applies in the same way to the other light
hydrocarbons mentioned above.
[0005] Depending on the composition of the natural gas, the
reforming process comprises the following steps 1 to 3: [0006] a
hot desulfurizing step 1, during which, after preheating of the
process natural gas to 300-400.degree. C., all the sulfur compounds
present in the natural gas are converted into H.sub.2S by a
catalytic process in a hydrogenation reactor, the H.sub.2S produced
being subsequently captured on a bed of adsorbent; [0007] an
optional pre-reforming step 2, during which the hydrocarbons
heavier than methane enter at a temperature of 450-550.degree. C.
in an adiabatic reactor, in which they are converted, under the
action of steam which is present, and in the presence of a
pre-reforming catalyst, into a mixture composed of CH.sub.4,
H.sub.2, CO and CO.sub.2; [0008] a reforming step 3, which involves
reacting the methane (CH.sub.4) with the steam contained in the
feedstock at high temperature, of the order of 850-950.degree. C.,
in the presence of a reforming catalyst, the reaction taking place
in tubular reactors which are installed in the furnace of a steam
reformer, to give H.sub.2, CO and CO.sub.2, the main constituents
of synthesis gas.
[0009] The subsequent steps are steps of treating the synthesis
gas, during which it undergoes changes in composition until the
desired end product is obtained.
[0010] Accordingly, the process for treating synthesis gas for
producing hydrogen will comprise some or all of the following steps
4 to 6: [0011] a step 4 of converting (also called shift or
water-gas shift reaction) the carbon monoxide present in the syngas
and steam into a mixture of hydrogen and carbon dioxide; this
conversion is performed in the presence of a shift catalyst in an
adiabatic reactor having an entry temperature of between
200.degree. C. and 360.degree. C. depending on the catalyst; this
step is carried out when required by the composition of the desired
synthesis gas, in particular when the main end product required is
hydrogen; [0012] a cooling/condensation step 5, during which the
synthesis gas enriched with H.sub.2 and CO.sub.2 is cooled in a
number of successive exchangers, and excess water is condensed and
separated from this gas; [0013] a step 6 of purifying the synthesis
gas in a pressure swing adsorption (PSA) unit to give hydrogen at a
purity of more than 99.99%, referred to as pure hydrogen, and a
residual gas mixture containing CH.sub.4, CO, H.sub.2 and CO.sub.2,
also referred to simply as "PSA residuals"; note that these PSA
residuals have a calorific value sufficient for recycling to the
burners of the reformer furnace in reforming step 3; an additional
fuel supply is provided by the fuel NG, both for supplementing the
energy supply from the residuals and for ensuring operational
flexibility.
[0014] The CO conversion reaction (water-gas shift reaction) of
step 4 is a catalytic equilibrium reaction. This reaction is
exothermic, and conversion of the CO is favoured at low
temperature. This reaction is conventionally carried out in an
adiabatic fixed-bed catalytic reactor.
[0015] Depending on its composition, the synthesis gas exiting the
conversion reactor is at a temperature which is higher than its
entry temperature by at least 50.degree. C. or even 150.degree. C.
or more; this increase in temperature owing to the conversion
reaction which takes place within the reactor at the same time
represents a conversion loss of the order of 10 to 15% relative to
a putative isothermal operation. A reactor which maintains a more
stable temperature--a cooled and ideally isothermal reactor--would
allow an increase in the conversion and therefore a reduction in
the consumption of process natural gas for a given level of
hydrogen production. On the other hand, increasing the conversion
in this reactor would lead to a reduction in the calorific value of
the PSA residues of step 6 that are recycled as a secondary fuel,
thereby necessitating compensation through an increase in the
supply of fuel NG.
[0016] There is therefore a need for a process which, for a given
level of ultimate production, allows a substantial reduction in the
overall consumption of the source gaseous hydrocarbons (natural
gas), in other words which makes it possible to reduce the
consumption of fuel NG at the same time as reducing the consumption
of process NG, so as to obtain ultimately a substantial reduction
in the overall consumption of NG.
[0017] The use of isothermal or pseudo-isothermal reactors for
carrying out exothermic catalytic reactions is known practice.
[0018] U.S. Pat. No. 7,981,271 B2, then, discloses a
pseudo-isothermal radial reactor which is used especially for the
synthesis of ammonia, and in which a plurality of exchangers in the
form of rectangular plates are immersed in a catalyst bed; the
stream of cold reactants acts as a heat transfer fluid, and the
reactants, after preheating in the plurality of exchangers,
traverse the catalyst bed radially.
[0019] US 2010/0176346 A1 for its part discloses an isothermal
reactor comprising tubes inserted in a shell. The shell contains
boiling water and the tubes are divided into two sections: in a
first section, the gas exiting reforming is circulated, and is
cooled by indirect exchange with the boiling water, whereas, in the
second section, which is filled with catalyst, the water-gas shift
reaction is performed. This second section is maintained at a
quasi-constant temperature by indirect cooling with the boiling
water, at least part of which is converted into steam.
[0020] US 2017/0021322 A1 discloses a pseudo-isothermal reactor for
carrying out exothermic reactions such as methanation or the
synthesis of methanol or formaldehyde. This reactor comprises two
catalytic zones which allow two-step conversion of the reactants.
These two catalytic zones are immersed in a single shell, in which
they exchange heat indirectly with boiling water or any other fluid
having a boiling point appropriate to the operating pressure of the
chamber.
[0021] These various documents teach the use of the exothermic heat
of a reaction either for preheating the reagents in the reaction
(U.S. Pat. No. 7,981,271) or for generating steam (US
2010/0176346-US 2017/0021322).
[0022] However, by increasing the temperature of the reactants
entering the reactor, the teaching of U.S. Pat. No. 7,981,271 B2
runs counter to the needs of the process to which the invention
applies, that process requiring the cooling of the synthesis gas
between steps 3 and 4; for their part, US 2017/0021322 A1 and US
2010/0176346 A1 use the heat of the water-gas conversion reaction
to heat water and thereby generate steam.
[0023] The performance data of prior-art conversion reactors used
for the water-gas shift reaction and integrated into the steam
reforming process in accordance with the prior art are presented in
[Table 1] later on below; the data show that: [0024] the
consumption of process NG decreases, but [0025] the conjoint
consumption of fuel NG increases, [0026] the resulting overall
hydrocarbon saving is limited, with environmental and economic
disadvantages and also disadvantages in terms of
return-on-investment period.
[0027] Accordingly, the solutions in the prior art do not provide a
satisfactory solution to the problem addressed, which is that of
reducing substantially the consumption of natural gas, both process
NG and fuel NG, for a given final production level.
SUMMARY OF THE INVENTION
[0028] It is therefore an objective of the invention to reduce
significantly the overall consumption of natural gas in the
process, this objective being achieved by reducing both the
consumption of process NG and the consumption of fuel NG, the first
through the use of a cooled reactor for converting the CO, and the
second by employing the heat, produced by the CO conversion
reaction and recovered in the reactor during this cooling, as a
heat supply in partial substitution for the fuel NG, for heating
the reforming feedstock. The solution of the invention therefore
allows a decrease in the overall consumption of natural gas by the
plant, without any change in the final production level.
[0029] To achieve this, the invention provides a process for
producing hydrogen from a light hydrocarbon source, comprising at
least the following steps:
[0030] step (a): generating a synthesis gas by steam reforming from
a source of said light hydrocarbons, this step itself comprising at
least a step (al) of desulfurizing said hydrocarbons for reforming,
an optional step (a2) of pre-reforming the desulfurized
hydrocarbons, a step (a3) of steam-reforming the desulfurized and
optionally pre-reformed hydrocarbons in tubular reactors installed
in the furnace of a reformer heated by burners which are fed with
fuel by at least secondary fuel gas, comprising some or all of the
residual gas mixture produced in the hydrogen production step (b)
of the process, and primary fuel, withdrawn preferably from the
light hydrocarbon source;
[0031] step (b): producing hydrogen from the synthesis gas
generated in step (a), itself comprising at least a step (bl) of
hydrogen-enriching the synthesis gas by conversion of carbon
monoxide according to the exothermic conversion reaction
CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2, and a step (b2) of purifying
the synthesis gas in a pressure swing adsorption unit to give a
pure H.sub.2 product and a residual gas mixture containing
CH.sub.4, CO, H.sub.2 and CO.sub.2,
[0032] characterized in that the conversion reaction of step (bl)
is carried out in a cooled conversion reactor in which some of the
heat produced in step (bl) is transferred by indirect heat
exchange--within the reactor--with a first fluid thus preheated
before it is used in the synthesis gas generation step (a), and in
that the hydrogen-enriched synthesis gas leaving the conversion
reactor is cooled by indirect heat exchange with a second fluid
before it is used in step (a).
[0033] The process of the invention may feature one or more of the
following variants: [0034] with the synthesis gas entering the
conversion reactor at a temperature T.sub.E, the hydrogen-enriched
synthesis gas leaves the convection reactor cooled to a temperature
T.sub.S of less than T.sub.E+40.degree. C., preferably less than
T.sub.E+30.degree. C., more preferably less than T.sub.E+10.degree.
C.; [0035] the first gaseous fluid to preheat is the residual gas
mixture feeding the burners of the reformer, and the second gaseous
fluid to preheat is composed of the light hydrocarbons for
reforming; [0036] the first gaseous fluid to preheat is composed of
the light hydrocarbons for reforming and the second gaseous fluid
to preheat is the residual gas mixture feeding the burners of the
reformer; [0037] for a hydrogen production level and operating
conditions which are otherwise similar, the total consumption of
light hydrocarbons is decreased by at least 1%, preferably at least
2%, more preferably still at least 3% relative to a process
employing an adiabatic conversion reactor; in other words, by
providing a total amount of light hydrocarbons decreased by at
least 1%, preferably at least 2%, more preferably still at least 3%
to the hydrogen production process, and maintaining other process
operating conditions similar, the hydrogen production level remains
unchanged.
[0038] According to another subject of the invention, the invention
provides a cooled conversion reactor for implementing any one of
the processes as defined above, characterized in that it is
equipped with means for admitting the first fluid to preheat before
it is used in step (a), reactor-internal means for circulating said
first fluid for preheating and for heat exchange with the synthesis
gas for cooling, and means for exit of said preheated first
fluid.
[0039] The reactor may feature one or more of the following
variants: [0040] the cooled conversion reactor may be a
plate-corrugated sheet reactor; [0041] the cooled conversion
reactor may be a shell-and-tube reactor, and may be adapted for
circulating synthesis gas in catalyst-filled tubes and for
circulating said first fluid for preheating in the shell; [0042]
the cooled conversion reactor may be a shell-and-tube reactor, and
may be adapted for circulating synthesis gas in the catalyst-filled
shell and for circulating said first fluid for preheating in the
tubes.
[0043] According to yet another subject of the invention, the
invention provides a plant suitable for implementing the process of
the invention according to any one of the embodiments described
above, characterized in that it is equipped with a cooled
conversion reactor selected from those described above, and is
equipped with means suitable for conveying said first fluid for
preheating and for bringing it to the said reactor, and with means
suitable for conveying said first preheated fluid from its exit
from said conversion reactor to the location of its use at the
reforming stage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] Further features and advantages of the invention will become
apparent from the description hereinafter of embodiments, which are
given by way of illustration but without any limitation, the
description being given in relation with the following attached
figures:
[0045] [FIG. 1] represents a schematic view presenting a process
for reforming natural gas for final production of hydrogen by PSA,
comprising, conventionally, an adiabatic reactor for the water-gas
shift reaction.
[0046] [FIG. 2] is a schematic view presenting a first solution
according to the invention, integrating a cooled reactor for the
water-gas shift reaction with a process for reforming natural gas
for final production of hydrogen by PSA, in which said reactor is
cooled via a cold source of the process.
[0047] [FIG. 3] is a schematic view presenting a second solution
according to the invention, integrating a cooled reactor for the
water-gas shift reaction with a process for reforming natural gas
for eventual production of hydrogen by PSA, in which said reactor
is cooled via a cold source of the process, different from the
first solution.
DETAILED DESCRIPTION OF THE INVENTION
[0048] The comparative performance data of prior-art processes and
processes according to the invention are presented in the tables
referenced below.
[0049] [Table 1] presents comparisons of respective performance
data of a process including an adiabatic reactor for the water-gas
shift reaction and of a process including a cooled reactor for this
same reaction according to the prior art, the conditions overall
being otherwise comparable.
[0050] [Table 2] presents comparisons of respective performance
data of a process including an adiabatic reactor for the water-gas
shift reaction and of a first process according to the invention
including a cooled reactor for this same reaction, the conditions
overall being otherwise comparable.
[0051] [Table 3] presents comparisons of respective performance
data of a process including an adiabatic reactor for the water-gas
shift reaction and of a second process according to the invention
including a cooled reactor for this same reaction, the conditions
overall being otherwise comparable.
[0052] The selected hydrocarbon for reforming is natural gas (NG);
other light hydrocarbon sources which may be reformed are suitable
in the same way, alone or in combination, including for the purpose
of providing the primary fuel. For the numbering of the elements
and fluids in the figures, three-digit numbers are used: the
hundreds digit corresponds to the reference of the figure, while
the two other digits identify the element or fluid referenced.
[0053] [FIG. 1] represents the conventional integration of an
adiabatic reactor for the water-gas shift reaction into the steam
reforming process. A mixture 104 composed of process natural gas,
which has been desulfurized and pre-reformed beforehand, and of
water vapour feeds the reforming tubes 102 which are present in the
reforming furnace 101, at a temperature of between 620.degree. C.
and 650.degree. C.; in contact with a steam reforming catalyst
which is present in the tubes, the hydrocarbons are converted and a
synthesis gas 108 exits at the bottom end of the tubes, where it is
collected. When it leaves the furnace, the synthesis gas is at a
temperature of the order of 900.degree. C. The burners 103 of the
furnace, which are intended to supply the tubes with the heat
required for reforming, are fed with combustion air 105, which is
preheated, and with fuel natural gas 106 and the residues from the
PSA unit 107, both of which are available at ambient temperature.
The very hot synthesis gas 108 exiting reforming is cooled to a
temperature of 360.degree. C. by heat exchange in a boiler 109 with
preheated water 110, thereby producing steam 111. At the boiler
exit, cooled synthesis gas 112 enters the adiabatic conversion
reactor 113 at 360.degree. C., where it undergoes the water-gas
shift reaction; some of the carbon monoxide present is converted
catalytically therein--via the water vapour present--into carbon
dioxide and hydrogen. The synthesis gas 114, enriched with H.sub.2
and CO.sub.2 , leaves the conversion reactor 113 at a temperature
higher than its entry temperature, of between 420.degree. C. and
430.degree. C., and subsequently gives up heat by passing through a
shell-and-tube heat exchanger 115, where it preheats process
natural gas 116 to a temperature of 360.degree. C. The preheated
process natural gas 117 feeds the desulfurization step of the
process (that step not being shown).
[0054] [FIG. 2] represents a first solution for
integration--according to the invention--of a cooled reactor for
the water-gas shift reaction. According to this solution, a mixture
204 of process natural gas which has beforehand been desulfurized
and pre-reformed, and of water vapour, feeds the tubes 202 of the
reforming furnace 201 at a temperature of between 620.degree. C.
and 650.degree. C.; on contact with a steam reforming catalyst
present in the tubes, the hydrocarbons are converted and a
synthesis gas 208 exits at the bottom end of the tubes, where it is
collected. On leaving the furnace, the synthesis gas 208 is at a
temperature of the order of 900.degree. C., and is subsequently
cooled to a temperature of 380.degree. C. in a boiler 209 which is
fed with preheated water 210, thereby producing steam 211. At the
boiler exit, the cooled synthesis gas 212 enters the cooled reactor
213 for the water-gas shift reaction; some of the carbon monoxide
present in the gas is converted in the presence of water vapour and
in contact with the catalyst which is placed in a fixed bed 214 in
the shell of the reactor 213, the conversion producing carbon
dioxide and hydrogen. A bundle of tubes 215 placed within the
catalyst bed allows the circulation of the residual PSA gases
207a--the hydrogen-purifying PSA unit is not shown--by circulating
in the tubes 215, the gases 207a are preheated; the preheated gases
207b attain a temperature of 357.degree. C. The synthesis gas 216,
enriched with H.sub.2 and CO.sub.2, is simultaneously cooled and
leaves the reactor at a temperature of 380.degree. C., and then is
used as a heat source in a shell-and-tube exchanger 217 for
preheating the process natural gas 218 to a temperature of
360.degree. C. The process natural gas thus preheated 219 feeds the
desulfurization step of the process--the latter step not being
shown.
[0055] The burners 203 of the reforming furnace 201 are fed with
preheated combustion air 205, with natural gas at ambient
temperature 206 and with the preheated residual gases from the PSA
unit 207b.
[0056] [FIG. 3] represents a second solution integrating, in
accordance with the invention, a cooled reactor for the water-gas
shift reaction into a process for production of synthesis gas by
steam reforming of natural gas. According to this solution, a
mixture 304 composed of process natural gas which has been
desulfurized--and if necessary pre-reformed--beforehand, and of
water vapour, enters the reforming tubes 302 of the steam reforming
furnace 301. The gas mixture, which enters at a temperature of
between 500.degree. C. and 650.degree. C., is contacted with a
steam reforming catalyst which is present in the tubes, and the
hydrocarbons are converted, the synthesis gas 308 being collected
at the exit from the tubes; the gas 308 leaving the steam reforming
furnace 301 is at a temperature of between 850.degree. C. and
950.degree. C. It is cooled to a temperature of between 250.degree.
C. and 400.degree. C. in a boiler 309 which is fed with preheated
water 310 and produces steam 311. The cooled synthesis gas 312 then
enters the cooled reactor 313 for the water-gas shift reaction.
Some of the carbon monoxide present is converted into carbon
dioxide and hydrogen in contact with the catalyst placed in a fixed
bed 314 in the shell. A bundle of tubes 315 is placed within the
catalyst bed, and a cooling fluid circulates therein, this fluid
being composed, according to this second solution example of the
invention, of the process natural gas 318 which must be preheated
before it enters the desulfurizing unit. The preheated natural gas
319 leaves the bundle of tubes 315 at a temperature of 360.degree.
C. and is sent to the desulfurizing unit (which is not shown).
[0057] After having given up some of its heat to the natural gas
318, the synthesis gas 316 exits the shift reactor 313 at a
temperature of 380.degree. C., and passes subsequently into a
shell-and-tube heat exchanger 317 , where it provides heat to the
residual gas 307a from the PSA unit. The PSA residuals 307b thus
preheated to a temperature of 360.degree. C. are sent to the
burners 303 of the steam reforming furnace 301. The burners 303 are
also fed with combustion air 305, which may or may not have been
preheated, and with fuel natural gas 306 at ambient
temperature.
[0058] The conversion reactors according to the invention that are
shown in [FIG. 2] and [FIG. 3] are shell-and-tube reactors, with
the synthesis gas circulating in the shell containing the catalyst,
and the gaseous fluid for reheating circulating in the tubes; it
would have been possible to employ a shell-and-tube reactor with
circulation of the synthesis gas in tubes containing catalyst and
with circulation of the fluid for reheating in the shell. It would
also have been possible to employ any other type of exchanger,
especially a plate-corrugated sheet exchanger.
[0059] [Table 1] below represents a comparison of the respective
performance data of a process including an adiabatic reactor for
the water-gas shift reaction and of a process including an
isothermal reactor for this same reaction, the following conditions
being otherwise comparable: [0060] the hydrogen production level is
the same for both configurations (5800 kmol/h); [0061] the
synthesis gas entering the conversion reactor has the same
composition in both cases: 49.3% H.sub.2, 10.2% CO, 5.2% CO.sub.2,
32.2% H.sub.2O, 0.3% N.sub.2, 2.8% CH.sub.4; [0062] in the first
configuration, the synthesis gas enters the adiabatic reactor (A)
at a temperature of 360.degree. C.; the exit temperature is then
428.degree. C.; [0063] in the second configuration, the isothermal
reactor (I) is itself operated at a temperature of 380.degree.
C.
TABLE-US-00001 [0063] TABLE 1 Adiabatic Isothermal Variation
reactor A reactor I (I-A)/A Consumption of 1972 1929 -2.2% process
NG (kmol/h) Consumption of fuel 296 327 +10.6% NG (kmol/h)
Consumption of total 2268 2257 -0.5% NG (kmol/h)
[0064] The performance data reported for the isothermal reactor are
the result of a simulation carried out on the basis of a
shell-and-tube reactor employing boiling water as cooling fluid.
The comparison of the respective performance data of a process
including an adiabatic reactor for the water-gas shift reaction and
of a process including an isothermal reactor (cooled, with
identical entry and exit temperatures of the natural gas) for this
same reaction shows that: [0065] with regard to the process NG: the
isothermal reactor offers better conversion, allowing a reduction
of 2.2% in the amount of process NG required to produce the 5800
kmol/h of hydrogen; [0066] with regard to the fuel NG: the PSA
residuals (step 6) which are used as secondary fuel are
nevertheless less CO-rich than in the case of the adiabatic
reactor, and there will therefore be a lower calorific supply,
which will have to be compensated otherwise, hence the increase in
the consumption of fuel NG by 10.6%; [0067] with regard to the
total NG: the result is an overall saving of 0.5% in the total
consumption of natural gas in a process employing an isothermal
reactor according to the prior art, relative to the standard
process employing an adiabatic reactor.
[0068] [Table 2] represents the comparative performance data of the
process of the invention shown in [FIG. 2], obtained by simulation,
and of the same, conventional process employing an adiabatic shift
reactor as in [Table 1]. In both cases, the hydrogen production
rate is the same, at 5800 kmol/h of hydrogen, and the synthesis gas
composition entering the reactor is the same: 49.3% H.sub.2, 10.2%
CO, 5.2% CO.sub.2, 32.2% H.sub.2O, 0.3% N.sub.2, and 2.8%
CH.sub.4.
TABLE-US-00002 TABLE 2 Adiabatic Isothermal Variation reactor A
reactor I (I-A)/A Entry temp. T.sub.E .degree. C. 360 380 Exit
temp. T.sub.S 428 380 .degree. C. CO content at 3.33 2.29 reactor
exit Temp. of PSA 35 357 residuals .degree. C. Consumption 1972
1932 -2.0% of process NG Consumption 296 261 -11.8% of fuel NG
Total 2268 2193 -3.3% consumption of NG Thermal yield 49.8 51.3 of
SMR furnace (%) Production of 73 60 -18.0% excess steam (t/h)
CO.sub.2 emitted 2479 2394 -3.4% (kmol/h)
[0069] The data presented show that: [0070] the PSA residuals,
initially at the temperature of 35.degree. C., are preheated in the
conversion reactor to a temperature of 357.degree. C., and exchange
their heat with the synthesis gas, which enters at the temperature
of 380.degree. C. and leaves at the same temperature despite the
exothermic heat of the conversion reaction; the amount of heat
which needs to be supplied to the reforming by the fuel NG is less,
and its consumption is therefore reduced; [0071] the preheating of
the PSA residuals gives rise to an increase in the thermal yield of
the reforming furnace (51.3% instead of 49.8%, or an increase of
1.5%), thereby reducing further the amount of heat which has to be
supplied and the consumption of fuel NG; [0072] the synthesis gas
exiting the conversion reactor is at a temperature very much lower
than that of the synthesis gas exiting the adiabatic conversion
reactor of the conventional process (380.degree. C. rather than
428.degree. C., or approximately 50.degree. C. lower), thereby
providing evidence of the stabilization of the temperature in the
conversion reactor, which allows an improvement in the yield of the
conversion reaction; consequently, the process NG flow rate
required for the same level of hydrogen production is reduced.
[0073] [Table 3] shows the comparative performance data of the
process of the invention according to [FIG. 3], as obtained by
simulation, and of the same, conventional process using an
adiabatic shift reactor as in [Table 1]. In both cases, the
hydrogen production rate of 5800 kmol/h of hydrogen is the same,
and the synthesis gas composition entering the reactor is the same:
49.3% H.sub.2, 10.2% CO, 5.2% CO.sub.2, 32.2% H.sub.2O, 0.3%
N.sub.2, 2.8% CH.sub.4.
TABLE-US-00003 TABLE 3 Adiabatic Isothermal Variation reactor A
reactor I (I-A)/A Entry temp. T.sub.E 360 370 (.degree. C.) Exit
temp. T.sub.S 428 380 (.degree. C.) CO content at 3.33 2.28 exit
from reactor Temp. of PSA 35 360 residuals (.degree. C.)
Consumption of 1972 1933 -2.0% process NG Consumption of fuel 296
261 -11.8% NG Total consumption of 2268 2194 -3.3% NG Thermal yield
of SMR 49.8 51.3 furnace (%) Production of excess 73 60 -18.0%
steam (t/h) CO2 emitted (kmol/h) 2479 2395 -3.4%
[0074] The data presented show that: [0075] the PSA residuals,
initially at a temperature of 35.degree. C., are preheated to a
temperature of 360.degree. C. by exchange of heat with the
synthesis gas having left the conversion reactor at a temperature
of 380.degree. C.; [0076] preheating of the PSA residuals produces
an increase in the thermal yield of the reforming furnace (51.3%
instead of 49.8%, or an increase of 1.5%), thereby further reducing
the amount of heat needing to be supplied and the consumption of
fuel NG; [0077] the synthesis gas exiting the conversion reactor is
at a temperature very much lower than that of the synthesis gas
exiting the adiabatic conversion reactor of the conventional
process (380.degree. C. rather than 428.degree. C., or
approximately 50.degree. C. lower), thereby providing evidence of
the stabilization of the temperature in the conversion reactor,
which allows an improvement in the yield of the conversion
reaction; consequently, the flow rate of process NG required for
the same level of hydrogen production is reduced.
[0078] In light of these tables, it is noted that the two
configurations according to the invention both feature comparable
advantages: [0079] reduction in the consumption both of process NG
and of fuel NG; for a given level of production, the consumption of
process NG is reduced by virtue of improved conversion in the
cooled reactor for the water-gas shift reaction, while the
consumption of fuel NG is reduced by virtue of a judicious
selection of the cooling fluid used for removing the heat produced
by the conversion reaction outside the conversion reactor; [0080]
because the latter is linked to the preheating of the PSA
residuals, it gives rise to an increase in the thermal yield of the
reforming furnace, thereby further reducing the amount of heat
which has to be supplied and the consumption of fuel NG; this
therefore signifies a greater reduction in the overall consumption
of natural gas, compared with the known solutions.
[0081] The two solutions proposed by the invention are
advantageous: [0082] firstly in terms of improved operation of the
reforming, as shown by Tables 2 and 3, with a decrease in the
consumption of hydrocarbons and an improvement in the thermal yield
of the reforming furnace; [0083] secondly because the shift reactor
operates at a lower temperature, therefore the sintering of the
catalyst will be reduced, hence increasing its lifetime.
[0084] Other criteria will allow one or other of the solutions to
be preferred, according to the case in hand.
[0085] The solution employing the PSA residuals as the heat
transfer fluid in the reactor is more favourable in the context of
updating an existing unit, since it involves fewer modifications to
the existing system: [0086] the piping of the natural gas
preheating system is unchanged; [0087] the exchanger for preheating
natural gas may stay the same or might require the addition of a
number of tubes, depending on its dimensions; [0088] the mechanical
integration of the unit is subject to few modifications apart from
for the conduits transporting the PSA residuals.
[0089] The solution employing natural gas as the heat transfer
fluid in the reactor will be preferred in the context of the
construction of a new unit, since the operation of the unit will be
made easier, the reasons being as follows: [0090] the start-up
sequence is simplified: in this solution, the heat transfer fluid
is available at start-up, whereas the PSA residuals (heat transfer
fluid in the solution of FIG. 2) are not (the entire chain upstream
of the PSA must first be started up, including the conversion
reactor). This means that this solution allows direct start-up of
the conversion reactor in cooled mode; [0091] for the same reason,
in the event of complete or partial failure of the PSA, there will
be no impact on the operation of the conversion reactor, by
contrast with the solution employing the PSA residuals for cooling
thereof.
[0092] The invention therefore presents many advantages relative to
the prior art and to the practice of the skilled person, among
those already cited: [0093] a reactor which maintains a more stable
temperature--a cooled and ideally isothermal reactor--allows an
increase in conversion and hence a reduction in the consumption of
process natural gas for a given level of hydrogen production;
[0094] the use of the heat produced by the conversion for heating
fluids feeding the reforming significantly decreases the total
consumption of hydrocarbons; [0095] the judicious use/removal of
the heat from the reactor via the preheating of the residues from
the PSA unit allows first some of the energy required for the
reforming to be supplied and second the thermal yield of the steam
reforming furnace to be increased.
[0096] This invention also leads to improvements in terms of the
plant considered within its environment, including the following:
[0097] since the consumption of natural gas goes down, the CO.sub.2
emissions will be decreased; [0098] the thermal yield of the
furnace increases, and the heat is used more effectively
internally, and so the production of excess steam will be
reduced.
[0099] While the invention has been described in conjunction with
specific embodiments thereof, it is evident that many alternatives,
modifications, and variations will be apparent to those skilled in
the art in light of the foregoing description. Accordingly, it is
intended to embrace all such alternatives, modifications, and
variations as fall within the spirit and broad scope of the
appended claims. The present invention may suitably comprise,
consist or consist essentially of the elements disclosed and may be
practiced in the absence of an element not disclosed. Furthermore,
if there is language referring to order, such as first and second,
it should be understood in an exemplary sense and not in a limiting
sense. For example, it can be recognized by those skilled in the
art that certain steps can be combined into a single step.
[0100] The singular forms "a", "an" and "the" include plural
referents, unless the context clearly dictates otherwise.
[0101] "Comprising" in a claim is an open transitional term which
means the subsequently identified claim elements are a nonexclusive
listing (i.e., anything else may be additionally included and
remain within the scope of "comprising"). "Comprising" as used
herein may be replaced by the more limited transitional terms
"consisting essentially of" and "consisting of" unless otherwise
indicated herein.
[0102] "Providing" in a claim is defined to mean furnishing,
supplying, making available, or preparing something. The step may
be performed by any actor in the absence of express language in the
claim to the contrary.
[0103] Optional or optionally means that the subsequently described
event or circumstances may or may not occur. The description
includes instances where the event or circumstance occurs and
instances where it does not occur.
[0104] Ranges may be expressed herein as from about one particular
value, and/or to about another particular value. When such a range
is expressed, it is to be understood that another embodiment is
from the one particular value and/or to the other particular value,
along with all combinations within said range.
[0105] All references identified herein are each hereby
incorporated by reference into this application in their
entireties, as well as for the specific information for which each
is cited.
* * * * *